Wafer producing method

ABSTRACT

A wafer is produced from a compound single crystal ingot having end surface. A separation plane is formed by setting the focal point of a laser beam inside the ingot at a predetermined depth from the end surface. The depth corresponds to the thickness of the wafer to be produced. The laser beam is applied to the end surface to form a modified layer parallel to the end surface and cracks extending from the modified layer, thus forming the separation plane. The ingot has first atoms having a larger atomic weight and second atoms having a smaller atomic weight, and the end surface of the ingot is set as a polar plane where the second atoms are arranged in forming the separation plane. After producing the wafer from the ingot, the first end surface is ground to be flattened.

This is a divisional of application Ser. No. 15/099,044, filed Apr. 14,2016.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer producing method for producinga wafer from a compound single crystal ingot.

Description of the Related Art

Various devices such as ICs and LSIs are formed by forming a functionallayer on the front side of a wafer formed of silicon or the like andpartitioning this functional layer into a plurality of regions along aplurality of crossing division lines. The division lines of the waferare processed by a processing apparatus such as a cutting apparatus anda laser processing apparatus to thereby divide the wafer into aplurality of individual device chips corresponding to the devices. Thedevice chips thus obtained are widely used in various electronicequipment such as mobile phones and personal computers. Further, powerdevices or optical devices such as LEDs and LDs are formed by forming afunctional layer on the front side of a wafer formed of a hexagonalsingle crystal such as SiC and GaN and partitioning this functionallayer into a plurality of regions along a plurality of crossing divisionlines.

In general, the wafer on which the devices are to be formed is producedby slicing an ingot with a wire saw. Both sides of the wafer obtainedabove are polished to a mirror finish (see Japanese Patent Laid-open No.2000-94221, for example). This wire saw is configured in such a mannerthat a single wire such as a piano wire having a diameter ofapproximately 100 μm to 300 μm is wound around many grooves formed onusually two to four guide rollers to form a plurality of cuttingportions spaced in parallel with a given pitch. The wire is operated torun in one direction or opposite directions, thereby slicing the ingotinto a plurality of wafers.

However, when the ingot is cut by the wire saw and both sides of eachwafer are polished to obtain the product, 70% to 80% of the ingot isdiscarded to cause a problem of poor economy. In particular, a compoundsingle crystal ingot of SiC or GaN, for example, has high Mohs hardness,and it is therefore difficult to cut this ingot with the wire saw.Accordingly, considerable time is required for cutting of the ingot,causing a reduction in productivity. That is, there is a problem inefficiently producing a wafer in this prior art.

A technique for solving this problem is described in Japanese PatentLaid-open No. 2013-49161. This technique includes the steps of settingthe focal point of a laser beam having a transmission wavelength to SiCinside a compound single crystal ingot, next applying the laser beam tothe ingot as scanning the laser beam on the ingot to thereby form amodified layer and cracks in a separation plane inside the ingot, andnext applying an external force to the ingot to thereby break the ingotalong the separation plane where the modified layer and the cracks areformed, thus separating a wafer from the ingot.

SUMMARY OF THE INVENTION

In the ingot cutting method described in Japanese Patent Laid-open No.2013-49161, however, the laser beam is applied again to an end surfaceof the ingot from which the wafer has been separated, thereby forming anew separation plane containing a modified layer and cracks.Accordingly, the end surface of the ingot must be ground to beflattened, so that there is a problem such that the wear amount(consumption) of abrasive members for grinding the end surface of theingot may become large and much time may be required for grinding,causing a reduction in productivity.

It is therefore an object of the present invention to provide a waferproducing method having a flattening step of grinding an end surface ofa compound single crystal ingot, wherein the wear amount of abrasivemembers can be suppressed and the grinding time can also be reduced.

In accordance with an aspect of the present invention, there is provideda wafer producing method for producing a wafer from a compound singlecrystal ingot having a first end surface and a second end surfaceopposite to the first end surface. The wafer producing method includes aseparation plane forming step of holding the second end surface of thecompound single crystal ingot on a chuck table, next setting a focalpoint of a laser beam having a transmission wavelength to the ingotinside the ingot at a predetermined depth from the first end surface ofthe ingot, which depth corresponds to a thickness of the wafer to beproduced, and next applying the laser beam to the first end surface asrelatively moving the focal point and the ingot to thereby form amodified layer parallel to the first end surface and cracks extendingfrom the modified layer, thus forming a separation plane containing themodified layer and the cracks; a wafer producing step of separating aplate-shaped member having a thickness corresponding to the thickness ofthe wafer from the ingot at the separation plane after performing theseparation plane forming step, thus producing the wafer from the ingot;and a flattening step of grinding the first end surface of the ingotafter performing the wafer producing step, thus flattening the front endsurface of the ingot. The ingot is composed of first atoms having alarger atomic weight and second atoms having a smaller atomic weight,the first end surface of the ingot being set as a polar plane where thesecond atoms having a smaller atomic weight are arranged in performingthe separation plane forming step. The first end surface as the polarplane where the second atoms having a smaller atomic weight are arrangedis ground in the flattening step.

Preferably, the compound single crystal ingot is a SiC ingot, and thefirst end surface is set as a polar plane where carbon (C) atoms arearranged. Alternatively, the compound single crystal ingot is a GaNingot, and the first end surface is set as a polar plane where nitrogen(N) atoms are arranged.

According to the wafer producing method of the present invention, theingot is composed of first atoms having a larger atomic weight andsecond atoms having a smaller atomic weight, and the first end surfaceof the ingot is set as a polar plane where the second atoms having asmaller atomic weight are arranged in performing the separation planeforming step. That is, the laser beam is applied to the first endsurface as the polar plane where the second atoms having a smalleratomic weight constituting the ingot are arranged. Further, in theflattening step, the first end surface as the polar plane where thesecond atoms having a smaller atomic weight are arranged is ground to beflattened. Accordingly, the wear amount of abrasive members in the caseof grinding the first end surface can be reduced to ½ to ⅓ as comparedwith the wear amount in the case of grinding the second end surface.Further, the required time for grinding in the case of grinding thefirst end surface can also be reduced to ½ to ⅓ as compared with thecase of grinding the second end surface.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus suitablefor use in performing the wafer producing method of the presentinvention;

FIG. 2 is a block diagram of a laser beam generating unit;

FIG. 3A is a perspective view of a compound single crystal ingot;

FIG. 3B is an elevational view of the compound single crystal ingotshown in FIG. 3A;

FIG. 4 is a perspective view for illustrating a separation plane formingstep;

FIG. 5 is a schematic sectional view for illustrating a modified layerforming step;

FIGS. 6A and 6B are perspective views for illustrating a wafer producingstep;

FIG. 7 is a perspective view of a wafer produced in the wafer producingstep;

FIG. 8 is a perspective view for illustrating a flattening step; and

FIG. 9 is a perspective view of the compound single crystal ingot heldon a chuck table in a grinding apparatus after performing the flatteningstep.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. Referring to FIG. 1, there isshown a perspective view of a laser processing apparatus 2 suitable foruse in performing the wafer producing method of the present invention.The laser processing apparatus 2 includes a stationary base 4 and afirst slide block 6 mounted on the stationary base 4 so as to be movablein the X direction. The first slide block 6 is moved in a feedingdirection, or in the X direction along a pair of guide rails 14 by afeeding mechanism 12 composed of a ball screw 8 and a pulse motor 10.

A second slide block 16 is mounted on the first slide block 6 so as tobe movable in the Y direction. The second slide block 16 is moved in anindexing direction, or in the Y direction along a pair of guide rails 24by an indexing mechanism 22 composed of a ball screw 18 and a pulsemotor 20. A support table (chuck table) 26 is mounted on the secondslide block 16. The support table 26 is movable in the X direction andthe Y direction by the feeding mechanism 12 and the indexing mechanism22 and also rotatable by a motor stored in the second slide block 16.

A column 28 is provided on the stationary base 4 so as to project upwardtherefrom. A laser beam applying mechanism (laser beam applying means)30 is mounted on the column 28. The laser beam applying mechanism 30 iscomposed of a casing 32, a laser beam generating unit 34 (see FIG. 2)stored in the casing 32, and focusing means (laser head) 36 mounted onthe front end of the casing 32. An imaging unit 38 having a microscopeand a camera is also mounted on the front end of the casing 32 so as tobe aligned with the focusing means 36 in the X direction.

As shown in FIG. 2, the laser beam generating unit 34 includes a laseroscillator 40 for generating a pulsed laser beam such as YAG laser andYVO4 laser, repetition frequency setting means 42 for setting therepetition frequency of the pulsed laser beam to be generated by thelaser oscillator 40, pulse width adjusting means 44 for adjusting thepulse width of the pulsed laser beam to be generated by the laseroscillator 40, and power adjusting means 46 for adjusting the power ofthe pulsed laser beam generated by the laser oscillator 40. Althoughespecially not shown, the laser oscillator 40 has a Brewster window, sothat the laser beam generated from the laser oscillator 40 is a laserbeam of linearly polarized light. After the power of the pulsed laserbeam is adjusted to a predetermined power by the power adjusting means46 of the laser beam generating unit 34, the pulsed laser beam isreflected by a mirror 48 included in the focusing means 36 and nextfocused by a focusing lens 50 included in the focusing means 36. Thefocusing lens 50 is positioned so that the pulsed laser beam is focusedinside a compound single crystal ingot 11 as a workpiece fixed to thesupport table 26.

Referring to FIG. 3A, there is shown a perspective view of the compoundsingle crystal ingot 11 as a workpiece to be processed. FIG. 3B is anelevational view of the compound single crystal ingot 11 shown in FIG.3A. The compound single crystal ingot (which will be hereinafterreferred to also simply as ingot) 11 is selected from a GaN singlecrystal ingot and a SiC single crystal ingot. The ingot 11 has a firstend surface 11 a and a second end surface 11 b opposite to the first endsurface 11 a. The first end surface 11 a of the ingot 11 ispreliminarily polished to a mirror finish because the laser beam isapplied to the first end surface 11 a.

The ingot 11 has a first orientation flat 13 and a second orientationflat 15 perpendicular to the first orientation flat 13. The length ofthe first orientation flat 13 is set longer than the length of thesecond orientation flat 15. The first end surface 11 a is a polar planein which the atoms having a smaller atomic weight constituting thecompound single crystal ingot 11 are arranged, wherein the ingot 11 iscomposed of the atoms having a larger atomic weight and the atoms havinga smaller atomic weight. Accordingly, the second end surface 11 b is apolar plane in which the atoms having a larger atomic weight arearranged.

In the case that the compound single crystal ingot 11 is a GaN ingot,the first end surface 11 a is a nitrogen (N) polar plane as −c planewhere the nitrogen atoms are arranged, whereas the second end surface 11b is a gallium (Ga) polar plane as +c plane where gallium atoms arearranged. On the other hand, in the case that the compound singlecrystal ingot 11 is a SiC ingot, the first end surface 11 a is a carbon(C) polar plane as −c plane where carbon atoms are arranged, whereas thesecond end surface 11 b is a silicon (Si) polar plane as +c plane wheresilicon atoms are arranged.

Referring again to FIG. 1, a column 52 is fixed to the left side of thestationary base 4. The column 52 is formed with a vertically elongatedopening 53, and a pressing mechanism 54 is vertically movably mounted tothe column 52 so as to project from the opening 53.

In performing the wafer producing method according to this preferredembodiment, the ingot 11 is fixed to the upper surface of the supporttable 26 by using a wax or adhesive in the condition where the first endsurface 11 a of the ingot 11 is oriented upward as shown in FIG. 4. Asmentioned above, the first end surface 11 a is a polar plane in whichthe atoms having a smaller atomic weight constituting the compoundsingle crystal ingot 11 are arranged, wherein the ingot 11 is composedof the atoms having a larger atomic weight and the atoms having asmaller atomic weight. In the case of a GaN ingot, the first end surface11 a is an N polar plane, whereas in the case of a SiC ingot, the firstend surface 11 a is a C polar plane.

After supporting the compound single crystal ingot 11 on the supporttable 26, a separation plane forming step is performed in such a mannerthat the focal point of a laser beam having a transmission wavelength(e.g., 1064 nm) to the compound single crystal ingot 11 fixed to thesupport table 26 is set inside the ingot 11 at a predetermined depth(shown by D1 in FIG. 5) from the first end surface 11 a, which depthcorresponds to the thickness of a wafer to be produced, and the laserbeam is next applied to the first end surface 11 a as relatively movingthe focal point and the ingot 11 to thereby form a modified layer 17parallel to the first end surface 11 a and cracks 19 propagating fromthe modified layer 17 as shown in FIG. 5, thus forming a separationplane.

This separation plane forming step includes a modified layer formingstep of relatively moving the focal point of the laser beam in the Xdirection to form the separation plane containing the modified layer 17and the cracks 19 propagating from the modified layer 17 inside theingot 11, and also includes an indexing step of relatively moving thefocal point in the Y direction to thereby index the focal point by apredetermined amount.

For example, the separation plane forming step in this preferredembodiment is performed under the following laser processing conditions.

Light source: Nd:YAG pulsed laser

Wavelength: 1064 nm

Repetition frequency: 80 kHz

Average power: 3.2 W

Pulse width: 4 ns

Spot diameter: 10 μm

Numerical aperture (NA) of focusing lens: 0.45

Index amount: 250 μm

In this manner, the focal point of the laser beam is sequentiallyindexed by the predetermined amount to form a plurality of modifiedlayers 17 at the depth D1 in the whole area of the ingot 11 and thecracks 19 extending from each modified layer 17 as shown in FIG. 5.Thereafter, a wafer producing step is performed in such a manner that anexternal force is applied to the ingot 11 to thereby separate aplate-shaped member having a thickness corresponding to the thickness ofthe wafer to be produced from the ingot 11 at the separation planecomposed of the modified layers 17 and the cracks 19, thus producing acompound single crystal wafer 21 shown in FIG. 7.

This wafer producing step is performed by using the pressing mechanism54 shown in FIG. 1. The configuration of the pressing mechanism 54 isshown in FIGS. 6A and 6B. The pressing mechanism 54 includes a head 56vertically movable by a moving mechanism (not shown) incorporated in thecolumn 52 shown in FIG. 1 and a pressing member 58 rotatable in thedirection shown by an arrow R in FIG. 6B with respect to the head 56. Asshown in FIG. 6A, the pressing mechanism 54 is relatively positionedabove the ingot 11 fixed to the support table 26. Thereafter, as shownin FIG. 6B, the head 56 is lowered until the pressing member 58 comesinto pressure contact with the first end surface 11 a of the ingot 11.

In the condition where the pressing member 58 is in pressure contactwith the first end surface 11 a of the ingot 11, the pressing member 58is rotated in the direction of the arrow R to thereby generate atorsional stress in the ingot 11. As a result, the ingot 11 is broken atthe separation plane where the modified layers 17 and the cracks 19 areformed. Accordingly, the compound single crystal wafer 21 shown in FIG.7 can be separated from the compound single crystal ingot 11.

After performing the wafer producing step, a flattening step isperformed in such a manner that the first end surface 11 a of thecompound single crystal ingot 11 from which the wafer 21 has beenseparated is ground to become flattened. In this flattening step, theingot 11 is held under suction on a chuck table 60 included in agrinding apparatus in the condition where the first end surface 11 a ofthe ingot 11 is oriented upward as shown in FIG. 8. That is, the secondend surface 11 b of the ingot 11 is held on the chuck table 60 undersuction. After holding the ingot 11 on the chuck table 60, a grindingunit 62 shown in FIG. 8 is used to grind the first end surface 11 a ofthe ingot 11. As shown in FIG. 8, the grinding unit 62 includes aspindle 64, a wheel mount 66 fixed to the lower end of the spindle 64,and a grinding wheel 68 detachably mounted on the lower surface of thewheel mount 66 by means of plural screws 67. The grinding wheel 68 iscomposed of a wheel base 70 and a plurality of abrasive members 72 fixedto the free end (lower end) of the wheel base 70 so as to be arrangedannularly.

In the flattening step, the chuck table 60 is rotated at 300 rpm, forexample, in the direction shown by an arrow a, and the grinding wheel 68is also rotated at 6000 rpm, for example, in the direction shown by anarrow b. Further, a grinding unit feeding mechanism (not shown) isdriven to lower the grinding unit 62 until the abrasive members 72 ofthe grinding wheel 68 come into contact with the first end surface 11 aof the ingot 11. Thereafter, the grinding wheel 68 is fed at apredetermined feed speed (e.g., 0.1 μm/second) to thereby grind thefirst end surface 11 a by a predetermined amount, thereby flattening thefirst end surface 11 a. Preferably, after performing the flatteningstep, the first end surface 11 a ground is polished to a mirror finish.

In this manner, the first end surface 11 a of the compound singlecrystal ingot 11 is flattened as shown in FIG. 9 by performing theflattening step, or preferably polished to a mirror finish aftergrinding. Thereafter, the separation plane forming step mentioned aboveis performed again. A comparison was made between the case that thefirst end surface 11 a is ground in the flattening step and the casethat the second end surface 11 b is ground in the flattening step.

Comparison Test (1)

Compound single crystal ingot: GaN ingot

Grinding amount: 5 μm

Grinding of Ga polar plane (second end surface 11 b)

-   -   Wear amount of abrasive members: 6.3 μm    -   Grinding time: 2.5 minutes

Grinding of N polar plane (first end surface 11 a)

-   -   Wear amount of abrasive members: 2.5 μm    -   Grinding time: 1 minute

Comparison Test (2)

Compound single crystal ingot: SiC ingot

Grinding amount: 5 μm

Grinding of Si polar plane (second end surface 11 b)

-   -   Wear amount of abrasive members: 7.5 μm    -   Grinding time: 3 minutes

Grinding of C polar plane (first end surface 11 a)

-   -   Wear amount of abrasive members: 3.5 μm    -   Grinding time: 1.5 minutes

According to the above preferred embodiment, the first end surface 11 aof the compound single crystal ingot 11 is a polar plane in which theatoms having a smaller atomic weight constituting the ingot 11 arearranged, wherein the ingot 11 is composed of the atoms having a largeratomic weight and the atoms having a smaller atomic weight. In theseparation plane forming step, the laser beam is applied to the firstend surface 11 a of the ingot 11 to form the separation plane inside theingot 11, wherein the separation plane is composed of the modifiedlayers 17 and the cracks 19. In the flattening step, the first endsurface 11 a as a polar plane where the atoms having a smaller atomicweight are arranged is ground to be flattened. Accordingly, the wearamount (consumption) of the abrasive members 72 in the case of grindingthe first end surface 11 a was reduced to ½ to ⅓ as compared with thewear amount in the case of grinding the second end surface 11 b asapparent from the above results of comparison. Further, the grindingtime (required time for grinding) in the case of grinding the first endsurface 11 a was also reduced to ½ to ⅓ as compared with the case ofgrinding the second end surface 11 b as apparent from the above resultsof comparison.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer producing method for producing a waferfrom a compound single crystal ingot having a first end surface and asecond end surface opposite to the first end surface, the waferproducing method comprising: a separation plane forming step of holdingthe second end surface of the compound single crystal ingot on a chucktable, next setting a focal point of a laser beam having a transmissionwavelength to the ingot inside the ingot at a predetermined depth fromthe first end surface of the ingot, which depth corresponds to athickness of the wafer to be produced, and next applying the laser beamto the first end surface as relatively moving the focal point and theingot to thereby form a modified layer parallel to the first end surfaceand cracks extending from the modified layer, thus forming a separationplane containing the modified layer and the cracks; a wafer producingstep of separating a plate-shaped member having a thicknesscorresponding to the thickness of the wafer from the ingot at theseparation plane after performing the separation plane forming step,thus producing the wafer from the ingot; and a flattening step ofgrinding the first end surface of the ingot after performing the waferproducing step, thus flattening the front end surface of the ingot,wherein the ingot is composed of first atoms having a larger atomicweight and second atoms having a smaller atomic weight, the first endsurface of the ingot being set as a polar plane where the second atomshaving a smaller atomic weight are arranged in performing the separationplane forming step, and the first end surface as the polar plane wherethe second atoms having a smaller atomic weight are arranged is groundin the flattening step.
 2. The wafer producing method according to claim1, wherein the compound single crystal ingot is a GaN ingot, and thefirst end surface is set as a polar plane where nitrogen (N) atoms arearranged.